1. Field of the Invention
The present invention generally relates to optical disk units, and more particularly to an optical disk unit that can record data on an optical disk.
2. Description of the Related Art
Optical disks of a direct-read-after-write type can be categorized into two types: a write-once type and an erasable type. Guiding grooves are formed on a compact disk recordable (CD-R), which is a write-once optical disk, and a compact disk rewritable (CD-RW), which is an erasable optical disk. The grooves wobble slightly in a radial direction at a center frequency of 22.05 kHz. Address information at the time of recording called Absolute Time In Pregroove (ATIP) is multiplexed and recorded in the grooves by frequency shift keying (FSK) with a maximum deviation of xc2x11 kHz.
In an optical disk unit for data recording and reproduction of such a recordable optical disk, a light beam is emitted onto the optical disk, the reflected light is detected by a plurality of photodetectors, a tracking error signal is generated by performing a given calculation, and a tracking actuator is driven based on the tracking error signal by a tracking servo circuit.
Here, in an optical disk unit for data recording and reproduction of the CD-R that is a write-once optical disk, the power of a light beam is set to a read power at the time of reproduction, while, at the time of recording, the power of the light beam alternates between a write power and the read power (write power greater than read power) in accordance with the value 0 and the value 1 of a recording signal. Therefore, at the time of not only reproduction but also recording, a tracking error signal is generated by sampling and holding a reflected light of a time at which the power of the light beam is set to the read power.
Further, in an optical disk unit for data recording and reproduction of the CD-RW that is an erasable optical disk, the power of a light beam alternates between a write power and an erase power (write power greater than erase power greater than read power) in accordance with the value 0 and the value 1 of a recording signal at the time of recording.
Therefore, at the time of reproduction, a tacking error signal is generated by detecting a reflected light of a time at which the power of the light beam is set to the read power, while, at the time of recording, the tracking signal is generated by sample-holding a reflected light of a time at which the power of the light beam is set to the erase power.
This is because a reflected light of a time at which the power of the light beam is set to the write power is unstable due to the effect of formation of recording regions, which are pits in the CD-R and marks in the CD-RW.
The differential push-pull method is one of the known tracking control methods. According to the differential push-pull method, a spot 2 of a main light beam (a main light beam spot 2) is formed on a groove 1 forming a track n and leading and trailing spots 3 and 4 of sub light beams (leading and trailing sub light beam spots 3 and 4) are formed in positions separated from the main light beam spot 2 by a given length in directions opposite to each other along the width of the groove 1 as shown in FIG. 1. The leading sub light beam spot 3 precedes the main light beam spot 2 in a scanning direction indicated by the arrow in FIG. 1. The trailing sub light beam spot 4 follows the main light beam spot 2 in the scanning direction. A reflected beam from the main light beam spot 2 is detected by two photodetectors 10A and 10B shown in FIG. 2 that are divided in the directions of the width of the groove 1 perpendicular to the scanning direction of FIG. 1. A reflected beam from the leading sub light beam spot 3 is detected by two photodetectors 12A and 12B that are divided in the width directions of the groove 1. A reflected beam from the trailing sub light beam 4 is detected by two photodetectors 14A and 14B that are divided in the width directions of the groove 1. The letters As and Bs of the light beam spots 2 through 4 of FIG. 1 correspond to the detectors 10A and 10B, 12A and 12B, and 14A and 14B of FIG. 2, respectively.
FIG. 2 is a diagram showing a configuration of a conventional tracking error signal generation circuit employing the differential push-pull method. In FIG. 2, the detection signals of the photodetectors 10A and 10B are supplied via a sample-and-hold circuit 16 to the non-inverting input terminal and the inverting input terminal of a subtractor circuit 18, respectively. A difference signal output from the subtractor circuit 18 is supplied to the non-inverting terminal of a subtractor circuit 20. The sample-and-hold circuit 16 samples the detection signals of the photodetectors 10A, 10B, 12A, 12B, 14A, and 14B at read-power or erase-power timings and holds the sampled detections signals during a write-power time (period).
The detection signals of the photodetectors 12A and 12B are supplied via the sample-and-hold circuit 16 to one input terminal of an adder circuit 22 and one input terminal of an adder circuit 24, respectively. The detection signals of the photodetectors 14A and 14B are supplied via the sample-and-hold circuit 16 to the other input terminal of the adder circuit 22 and the other input terminal of the adder circuit 24, respectively. The adder circuit 22 adds the two supplied signals and supplies an output signal to the non-inverting input terminal of a subtractor circuit 26. The adder circuit 24 adds the two supplied signals and supplies an output signal to the inverting input terminal of the subtractor circuit 26. A difference signal output from the subtractor circuit 26 is amplified by a gain k in an amplifier 28 for the differential push-pull method and supplied to the inverting input terminal of the subtractor circuit 20.
An error signal output from the subtractor circuit 20 is supplied to an adder 30, which adds an offset at the time of reproduction or recording (a reproduction or recording offset) supplied from a switch 32 to the error signal. Thereby, the error signal is output from a terminal 34 as a tracking error signal (TES). By performing tracking control so that the tracking error signal is caused to be zero, tracking is performed so that the main light beam spot 2 follows the center of the groove 1.
The read power and the write power, and further, the erase power and the write power of a laser light (a light beam) emitted from a laser diode are different so as to cause a shift of the optical axis of the laser light. As shown in FIG. 3, a laser light emitted from a laser diode 36 is as indicated by a solid line at a read-power or erase-power time and as indicated by a broken line at a write-power time, thus causing a shift of the optical axis of the laser light by an angle xcex8. If the shift of the optical axis is caused in the width directions of the groove 1, recording regions (pits in the case of the CD-R) formed by the write power of the laser light are deviated from the center of the groove 1 since the tracking error signal is also generated at the read-power or erase-power time at the time of recording. In order to correct this deviation of the recording regions, the switch 32 is provided to perform switching between the reproduction offset and the recording offset. During recording, the switch 32 remains switched to the recording-offset side all the time.
In the production process of the conventional optical disk unit, a signal is actually recorded on an optical disk and the recorded signal is reproduced. Thereby, the offset of the tracking error signal at the time of recording is detected and maintained. However, if a change in the characteristic of the laser diode 36 is caused by a change in temperature or the passage of time so as to effect a change in the amount of a shift of the optical axis from its position at the read-power time to its position at the write-power time, a tracking deviation is caused at the time of recording so as to prevent accurate tracking. This is because the fixed recording offset detected in the production process is employed.
When the fixed recording offset is always added to the tracking error signal generated at the read-power time as in the conventional case, the light beam may be focused into a spot whose center is on a track at the write-power time. However, at the read-power time, the light beam is focused into a spot whose center is deviated from the track since the recording offset is also added to the tracking error signal at the read-power time. This causes the problem of reduction in the accuracy of reading ATIP information (address information) obtained from the reflected light that is detected at the read-power time.
Accordingly, it is a general object of the present invention to provide an optical disk unit in which the above-described disadvantages are eliminated.
A more specific object of the present invention is to provide an optical disk unit that realizes reduction in an offset caused by the shift of the optical axis and an increase in the accuracy of reading ATIP information with a simple configuration.
The above objects of the present invention are achieved by an optical disk unit performing tracking control by generating a tracking error signal based on detection signals of reflected lights of a main light beam for recording and reproduction and sub light beams for tracking, the main light beam being focused into a spot centered on a center of a track of an optical disk, the sub light beams being focused into spots whose centers are offset from the center of the track in directions of a width of the track, the optical disk unit including a first tracking error generation part generating a first tracking error component from the detection signals of the reflected light of the main light beam of a read-power time or an erase-power time, a second tracking error generation part generating a second tracking error component from the detection signals of the reflected lights of the sub light beams of a write-power time, and a subtraction part obtaining the tracking error signal for a recording time by subtracting the second tracking error component from the first tracking error component.
The above objects of the present invention are also achieved by an optical disk unit performing tracking control by generating a tracking error signal based on detection signals of reflected lights of a main light beam for recording and reproduction and sub light beams for tracking, the main light beam being focused into a spot centered on a center of a track of an optical disk, the sub light beams being focused into spots whose centers are offset from the center of the track in directions of a width of the track, the optical disk unit including a first tracking error generation part generating a first tracking error component from the detection signals of the reflected light of the main light beam of a read-power time or an erase-power time, a second tracking error generation part generating a second tracking error component from the detection signals of the reflected lights of the sub light beams of all times of respective power levels, and a subtraction part obtaining the tracking error signal for a recording time by subtracting the second tracking error component from the first error component.
According to any of the above-described optical disk units, by using the fact that the reflected light of the main light beam shifts in a direction opposite to a direction in which the reflected light of each sub light beam shifts when the laser beam is switched from the read power to the write power, an offset resulting from the shift of the optical axis of the main light beam from its position at the read-power time (erase-power time) to its position at the write-power time at the time of recording can be reduced with a simple configuration. Further, an offset correction is performed so that the main light beam is focused to be centered on the track center at each of the read-power time (erase-power time) and the write-power time. This not only improves recording quality but also increases the accuracy of reading ATIP information detected from the reflected light of the main light beam at the read-power or erase-power timings at the time of recording.